The invention concerns a procedure for error correction by the de-embedding of a scattering parameter, which has been measured with an n-port containing, vectorial network analyzer, wherein the scattering parameter relates to a device under test connected to the said ports. The invention also concerns a vectorial network analyzer for the said procedure as well as a circuit module for the said network analyzer.
In high frequency technology, the behavior of circuits is normally described in terms of scattering parameters. The scattering parameters represent complex reflection and transmission parameters of a circuit and join ingoing and outgoing waves with one another. A representation of this type where complex reflection and transmission parameters are concerned is especially well suited to the physical realities of the problems brought forth in high frequency technology.
Thus, for example, a circuit, which is formed by a linear 2-port, and is incorporated in the scattering parameters by means of its scattering matrix [S], can be completely described. If the waves, which respectively run to one port of the 2-port, are designated as a1 and a2 and those waves which depart from respectively one port of the 2-port, and propagate themselves in a reverse direction, are designated by b1 and b2, then, for the scattering matrix [S] the following valid relationship serves:
      (                                        b            1                                                            b            2                                )    =                    (                                                            S                11                                                                    S                12                                                                                        S                21                                                                    S                22                                                    )            ⁢              (                                                            a                1                                                                                        a                2                                                    )                    ︸              =                  [          S          ]                    Experience in the practice allows it to be known, that for the determination of the scattering parameter, it is advantageous to employ a circuit of a network analyzer to which the circuit of the device under test can be connected. By means of such a network analyzer, the waves approaching the device under test are input and captured at the measurement positions. Likewise, the waves sent in the opposite direction are captured at measurement positions. From these measured values, it is then possible to determine the scattering matrix [S].
The goal of every n-port measurement by means of a network analyzer is to determine the scattering parameter with the greatest precision. In any case, error interferences occur throughout the network analyzer itself, such as, for example, improper interlinkage or mismatching, which falsify the results of measurements.
The precision of the measuring capacity of the network analyzer, can be substantially improved by a system error correction. Where the system error correction is concerned, measurement takes place within a calibration process, this being the so-called calibration standards, that are devices under test, which are partially or fully known. From these measurement values and through special computational paths, one obtains correction data. With these correction data and a corresponding correction computation, one obtains for each device under test from the rough measurement values, corrected scattering parameters, which are then free from the said system error of the network analyzer.
De-embedding is to be understood as a situation wherein after a calibration has been made as described, one has obtained scattering parameters, which are not yet sufficiently error-free, and subsequently the said scattering parameters are subjected to a second measurement correction. This can be, in the simplest case, a multiplication with an inverse chain matrix of a known circuit line between the network analyzer and the device under test. As a rule, a good calibration in the reference planes of the device under test is more exact than an additional de-embedding step. However, a calibration is often time consuming and complex, and in many cases the exactness of a de-embedding step is sufficient. In the literature, de-embedding is designated as a “Two-Tier-Calibration”, which also makes clear, that when de-embedding is practiced, a two-stage calibration, i.e. a two-stage error-corrective measure of the raw measured values is being undertaken.
Principally, network analyzers are a means of measuring electronic equipment of one and 2-port parameters in a range of, for instance, electronic semiconductor components to antennas. These 1-port and 2-port calibration procedures form, however, no fully sufficient basis for error correction in the measurement of multiport objects. One problem with multiport measurement is found therein, in that, namely all ports of the device under test are interlinked.
Thus, one cannot obtain, from a single point of measurement, a value for the waves departing, then at the next measuring point, achieve a value for the reflected wave, and finally at a third measurement point, pick up a value for a transmitted wave, which value would be independent of the connection terminals of the multiport device.
However, for several years, network analyzers with a nearly optionally large number n of measuring ports have been put to use for the detection of the complex reflection and transmission characteristics of multiport devices under test. Procedures in accord with this have been described in the documents DE 199 18 697 A1 and DE 199 18 960 A1. DE 199 18 960 A1 is based on the use of a 7-term-procedure for 2-port measuring, and DE 199 18 697 A1 is based on the use of a 10-term procedure for 2-port measuring. The calibration procedures presented in the said documentation for the error module for n-port network analyzers are direct multiport calibration procedures, which, to be sure, are exact, but however, with which, in practice, the necessary measurements and corrections are very time consuming. The result of this is, that these procedures cannot be employed for network analyzers, which exhibit three or four measurement positions, which by means of two inner ports and a circuit matrix are connected with the ports of the device under test. These network analyzers present, however, by far the largest group of applied network analyzers.
In modern network analyzers with four measuring positions, there has been one in the dissertation publication “Safe Procedure for the Calibration of Network Analyzers for Coaxial and Planar Line Systems”, Institute for High Frequency Technology, Ruhr University, Bochum, 1995, from H. Heuermann's descriptive TRL, i.e. “Through Line Reflectance Calibration Procedure”. In this procedure, there is required, aside from the through connection T, the remaining two standards L and R, which need be only partially known. In the said publication, however, it has been shown, that the TRL-procedure can be seen as essentially a special case of a general theory for the so-called “two error matrix 2-port model”.
As devices under test of multiport measurements, first, there is a series of objects with unsymmetrical terminal connections (as a rule, 50 Ω-ports) such as couplers, signal parts, and frequency selective filters, and second, objects to be considered with connection terminals of various types, such as, for instance, symmetrical members and SAW-filters (i.e., Surface Acoustic Wave filters). In the case of the latter, the state of the technology is, that the differential mode, by means of an additional working step under the limitations of an ideal transformer is retroconverted into an unsymmetrical mode.
Thus, the invention has the purpose of making available a procedure, a network analyzer and a circuit module, which permit a universally applicable, exact and non-time consuming error correction of the scattering parameters of a device under test, as measured by means of a network analyzer.